The stability of neuronal form increases during neuronal development. The experiments of this proposal are based on the hypothesis that the increase in the stability of neuronal form reflects a corresponding increase in the stability of the cytoskeleton. A clonal line of pheochromocytoma (designated PC12) and cultures of fetal rat sympathetic neurons will be used as model systems to study changes in microtubule stability during neurite outgrowth and the maturation of neurite structure. Neurites of developing neurons are dependent on microtubules for the maintenance of their structural integrity. Thus, it is likely the stability of microtubules will strongly influence the stability of neurite form. Preliminary studies with PC12 cells suggest that the stability of microtubules increases during neurite outgrowth. The proposed project will extend these studies by using pharmacological and biochemical procedures to assay microtubule stability in differentiating PC12 cells and in normal differentiating neurons. Cellular mechanisms which are involved in regulating microtubule stability during neurite outgrowth and maturation will be investigated by determining whether quantitative and/or qualitative changes occur in the composition of microtubule associated proteins and in the subunit composition of tubulin during growth and maturation of neurites. The results of these studies will provide useful information concerning the regulation of microtubule stability during neuronal development. Because of the importance of microtubules in maintaining neuronal form, knowledge of the mechanisms involved in the regulation of microtubule stability will provide a more detailed understanding of the cellular mechanisms involved with the production and stabilization of neuronal architecture.